migrate PR template

.forgejo/pull_request_template.md

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+<!-- ℹ お読みください / README
+PRありがとうございます！ PRを作成する前に、コントリビューションガイドをご確認ください:
+Thank you for your PR! Before creating a PR, please check the contribution guide:
+https://github.com/misskey-dev/misskey/blob/develop/CONTRIBUTING.md
+-->
+	
+## What
+<!-- このPRで何をしたのか？ どう変わるのか？ -->
+<!-- What did you do with this PR? How will it change things? -->
+	
+## Why
+<!-- なぜそうするのか？ どういう意図なのか？ 何が困っているのか？ -->
+<!-- Why do you do it? What are your intentions? What is the problem? -->
+	
+## Additional info (optional)
+<!-- テスト観点など -->
+<!-- Test perspective, etc -->
+	
+## Checklist
+- [ ] Read the [contribution guide](https://github.com/misskey-dev/misskey/blob/develop/CONTRIBUTING.md)
+- [ ] Test working in a local environment
+- [ ] (If needed) Add story of storybook
+- [ ] (If needed) Update CHANGELOG.md
+- [ ] (If possible) Add tests